On the role of Black Sea Waters in controlling the North Aegean buoyancy fluxes

Understanding the processes that control the buoyancy fluxes of the Aegean Sea is important for various reasons. First, the Aegean is directly connected with the Black Sea and acts as a buffer between two opposite thermohaline systems, a concentration and a dilution basin (the Mediterranean vs the Black Sea), receiving and filtering the variability and changes of a much broader geographical area. Second, the Aegean is capable to produce large amounts of very dense water, having temporarily been the major originator of Eastern Mediterranean Bottom Water. These processes are controlled by the buoyancy fluxes, both through oceanic advection and atmospheric exchanges. In this work we examine the characteristics and variability of heat, freshwater and the overall buoyancy air-sea fluxes, focusing on the potential role of the interaction with the Black Sea. A thirty-year-long simulation (1985-2015) of the whole Eastern Mediterranean/Black Sea system, forced by ERA-Interim data, was used to estimate and analyze the seasonal and interannual variability of the buoyancy fluxes in the North Aegean. The climatological mean buoyancy flux over the North Aegean has been estimated to be about –10×10–6 kg m–1 s–3 (loss to the atmosphere). However, in the absence of Black Sea Water (BSW) inflow, the buoyancy loss would correspond to the high values observed over the Eastern Aegean, an area not directly affected by the presence of the BSW, i.e. about –30 ×10–6 kg m–1 s–3. It should be noted that the heat loss of the Aegean Sea to the atmosphere is much higher than all neighboring seas, including the Adriatic, the dominant dense-water formation site for the Eastern Mediterranean. Our analysis reveals that the thin surface layer of modified BSW acts as a moderator of the buoyancy loss from the upper water column. This layer not only absorbs the air-sea fluxes (acting as an effective insulator regarding dense-water formation processes), but also moderates or even reverses the buoyancy fluxes over its path. Thus, in addition to the significant lateral buoyancy input to the basin by the Black Sea inflow, an additional mechanism of reduction of winter heat losses to the atmosphere contributes to the control of dense water formation processes.